Field of the Invention
The present invention relates to methods and compositions for inhibiting the formation of naphthenate precipitates or naphthenate-stabilized emulsions during contact between crude oil containing naphthenic acid and water containing cations.
Background of the Related Art
Crude oil includes various high molecular weight hydrocarbon-containing compounds, frequently including naphthenic acid. Naphthenic acids are classified as carboxylic acids of the general formula R—COOH, where R represents a cyclo-aliphatic structure typically having 10 to 90 carbon atoms and 0 to 6 rings. The term “naphthenic acid” is generally used to account for carboxylic acids present in crude oil, including acyclic and aromatic acids. Naphthenic acids are found predominantly in immature biodegraded crude oils. A special type of naphthenic acid was discovered in 2005, this naphthenic acid has a molecular weight of 1231 Dalton, and has 4 carboxylic groups per molecule. The molecule is often referred as naphthenic tetra acid (TA), tetraprotic acid and also ARN acid. This discovery has lead to a further analysis of naphthenic acid and the discovery of di, tri and tetra acid in crude oil samples. The amount of organic acid present in crude oil is expressed by the total acid number (TAN), often in units of milligrams KOH per gram of crude. For example, crude oil produced from the North Sea, the Far East and Western Africa exhibit high TAN numbers, such as a TAN greater than one (1), including high concentrations of naphthenic acids.
One problem experienced during production of crude oils having a high TAN number and in particular a high tetra acid content, is the formation of calcium naphthenate precipitates and/or naphthenate stabilized emulsions. The ionizable carboxylic acid group present in naphthenic acids exhibits a hydrophilic nature that causes the naphthenic acid molecule to congregate at the oil-water interface. In particular, the four carboxylic acid groups in the naphthenic tetra acid can react to form a network of tetra acid crosslinked by calcium. At low pH (acidic) conditions, the interfacial activity of the naphthenic acids is not so problematic because the predominant species is the charge-neutral carboxylic acid. As the crude oil is produced, the pressure in the crude oil drops and carbon dioxide is lost from the solution. The loss of carbon dioxide causes the pH of the crude oil to increase (become basic) and, in turn, leads to enhanced dissociation of the naphthenic acid. The resulting naphthenates can then act as natural surfactants leading either to stabilized emulsions or solid deposits following complexation with calcium or other cations present in the aqueous phase. The naphthenate deposits can accumulate in flow-lines, heat-exchangers, oil-water separators, desalters, filters, hydro-cyclones, and other oil processing equipment.
When naphthenic acids in crude oil are combined with high-pH (basic), highly mineralized connate or process waters, the processes that separate oil and water can experience severe problems. The neutralization products of naphthenic acids with basic ions in the water tend to form very stable water-in-oil emulsions and/or insoluble sticky calcium naphthenate deposits. The naphthenate deposits are similar in nature to lime soaps that form from fatty carboxylates and calcium chloride, sometimes known as “soap scum.” These phenomena can hamper the oil production considerably and result in high treatment costs.
Existing treatments to prevent the formation of calcium naphthenate precipitates and emulsions during oil production include the injection of large volumes of acetic acid in order to decrease the pH of the oil and water phases, thus favoring naphthenic acid over the formation of naphthenate salts. However, a continuous injection of about 100 to 1000 parts per million (ppm) of acetic acid is required to achieve a pH below at least 6.0 and sometimes even below 5.2.
However, this acid is very corrosive and presents various health, safety, and environmental issues. Furthermore, the sheer volume of acetic acid solution that is necessary represents a significant storage and supply problem, especially on offshore oil production platforms. Also, lowering the pH of the produced waters may lead to serious internal corrosion of the crude oil processing equipment. The result is that preventing the formation of naphthenate precipitates and emulsions comes at a high cost.
Publication WO 2006/025912 A2 describes a method of inhibiting the formation of naphthenate precipitates and emulsions during the production of crude oil. The low dosage naphthenate inhibitors require smaller volumes of active chemical, lower dosage rates, smaller pH changes, and lower overall costs. The inhibitor composition is added to a mixture of water and oil in an amount that is effective to inhibit interaction between cations in the water and organic acid in the oil that can lead to the formation of organic salts. The cations originating from the water, such as connate water, formation water or seawater, may include an inorganic cationic species, such as calcium cations, magnesium cations, sodium cations, or combinations thereof. The organic acid originating from the oil may include a naphthenic acid. The inhibitor compositions are suitable for use with oil having a total acid number (TAN) greater than 0.05 and even a TAN greater than 1.
Should any metal naphthenate salts already exist prior to adding the inhibitor or form despite the presence of the inhibitor, the inhibitor composition may additionally inhibit the agglomeration of these reaction products of metal ions and organic acids in the oil via crystal habit modification. The term “crystal habit” refers to the typical course of events that occur when a material becomes insoluble by forming a specific type of crystal structure. One type of crystal habit modification can be brought about by adding inhibitor molecules that interfere with crystallization to the extent that the crystals are unstable, do not agglomerate, and therefore do not form deposits on oil production equipment.
The inhibitor composition inhibits the cations and organic acid, such as a naphthenic acid, from interacting along an oil-water interface, such as an oil-water emulsion. Preferably, the inhibitor composition inhibits formation of an organic salt, such as a naphthenate salt, along an oil-water interface. More specifically and preferably, the inhibitor composition inhibits naphthenate salt formation along an oil-water interface. Accordingly, the inhibitor composition may have a hydrophilic portion and a lipophilic portion.
The above methods are capable of inhibiting calcium naphthenate formation, at low dosage and without decreasing the produced water pH to the corrosion envelope. Field experience and lab testing show that the inhibitor residence time on the oil/water interface may be relatively short (10-60 min). Although this can be mitigated by the use of a fast emulsion resolution and fast dehydration to decrease the interface surface area, supplemental treatment might be required in some cases. In addition at high pH and longer residence time, calcium naphthenate might still be found, when left untreated.
An important feature of the low dose naphthenate inhibitor is that these inhibitors do not influence the dehydration process. This is achieved by selecting hydrotropes that do have an affinity for oil/water interface orientation, however, do not have a strong surfactant behavior. The hydrophilic-lipophilic balance (HLB) contrast of the selected material is rather low. By contrast, dodecylbenzene sulfonic acid (DDBSA) is a strong surfactant and also a good naphthenate inhibitor. However, because of the high surfactant, emulsifying behavior, DDBSA influences the dehydration process and actually has a detrimental effect on the required time to dewatering of the crude. In most offshore installations, the residence time is too short for DDBSA to be used as inhibitor.